Engine control systems require accurate control of the air/fuel ratio for controlling regulated emissions. In addition fuel economy improvements from operating the engine in the lean region also require precise air/fuel ratio control. For precise air/fuel control, an open loop estimate of the air entering the engine is required due to the delayed exhaust sensor response. The open loop estimate of air entering the cylinder, or cylinder air charge, is a particular problem when using a mass air flow sensor. The problem is due to the manifold volume, which tends to add dynamics that separate the air measured at the throttle body by the mass air flow sensor and from air inducted into the cylinder.
One approach to estimating the cylinder air charge is to use a first order difference equation that represents the manifold dynamics. In this system, the difference equation parameter, or filter parameter, is a function of volumetric efficiency, which is in turn a function of atmospheric conditions. For example, the filter parameter is a function of barometric pressure and air temperature. Each of these parameters is constantly monitored and used to recalculate the filter parameter during every operation of the estimation method. In addition the difference equation uses past parameters, thus requiring intermediate parameter storage. Such a system is described in U.S. Pat. No. 5,331,936.
The inventors herein have recognized a disadvantage with the above system. The nature of the difference equation requires extensive testing and parameter mapping across many conditions, such as, for example, changes in barometric pressure and air temperature. As a result, calibration effort of such an estimation scheme is greater than necessary. In addition because the parameter values are functions of many other parameters, extensive computation power is required for performing all of the necessary computations. Further, past parameter storage requires additional memory and computer operation time.